1. Field of the Invention
The present invention relates to printing techniques in general and to what is known as halftone printing in particular. More particularly still, it relates to printing with more than a single printing plate. The preferred embodiment of the present invention is directed to printing on metal cans or the like, also known as metal decorating.
2. Description of the Prior Art
U.S. Pat. No. 3,948,171 granted Apr. 6, 1976 to Richard E. O'Connell and titled: Method For Printing Multicolor Halftone Images on Cylindrical Objects provides a novel method and apparatus for producing halftone effect printing using line-engraves press plates for printing cylindrical objects, particularly cans of the two-piece type. The patent, under the heading "Description of the Prior Art" provides a brief but useful discussion of problems in multicolor printing on two-piece cans at that time. Beginning at Column 1, line 36 the patent states:
"The printing techniques that have been developed for this so-called two-piece can construction are necessarily modified to provide for offset printing of multicolor images by direct application to the cylindrical surface. The methods employed include the formation of printing plates for each of several colors to be applied to the work. The traditional halftone lithograph effect by the use of printing mats prepared by the use of color separation, halftone dot or grid type screens and press plates, and overprinting have not been attainable in two-piece can printing since the problems of securing registry, proper printing pressure, angle of screen and plate, and the like to achieve the desired halftone effect attained by overprinting using halftone plates is, as a practical matter, difficult to accomplish. PA0 "The traditional grid or dot type screens used to form the halftone printing plates for multicolor printing produces a plate in which the printing surface is, in effect, a plurality of tiny raised dots or islands. The amount of color applied by each of the halftone plates was determined by the presence of and size of each of these dots, each of which, in turn, were formed photographically by traditional color separation and photoengraving techniques using appropriate filters and a halftone screen or grid. For each color, the grid and the resulting press plate is angled slightly to result in partial overprinting. The final composite multicolor shaded or toned printing product is achieved by overprinting and by optical blending, producing a visual effect of the desired tones or blends of color. Since each of the small dots formed in the press plate are fragile, the mechanical stress encountered in the application of each color to the offset surface or blanket results in smearing of the individual dots by the mechanical bending action encountered in the contact between the press plate and the surface of the offset blanket. This is especially true in the preparation of so-called flexible halftone press plates where the plate is formed by a chemical photoengraving method. In this procedure the plate is formed of a flexible, rubbery or polymeric material which has a photosensitive surface. On exposure to light, the photosensitive material is activated or hardened so that further treatment or etching, as by solvent, dissolves away the unexposed portions leaving a raised printing surface formed of a multitude of dots which corresponds to the particular color image applied to the plate. Since the base supporting the raised or printing portion is flexible, each printing operation flexes each dot and tends to smear the image and can also break the dots off. PA0 "Understandably, the quality of the images will deteriorate rapidly under these stresses, and such indeed is the experience in the can lithographing art. Likewise, the achievement of tone or color blend by traditional offset multicolor halftone techniques requires very accurate registry, careful angulation of the screens and press plates, accurate pressures, and more particularly, evenness of printing pressures. These desiderata were not attainable in the high speed printing techniques used for cylindrical objects or cans and required for economic operation in this industry. The result has been that halftone effects are not reproducible by any of the known standard multicolor offset or lithographic printing techniques which have been employed to date in the printing of two-piece cans. PA0 "Some of the prior art patents which deal with high speed can printing of two-piece metal cans include: Brigham, et al., U.S. Pat. No. 3,227,070; Brigham, U.S. Pat. Nos. 3,223,028 and 3,371,603; Freres, U.S. Pat. No. 3,385,209; and Maxin, U.S. Pat. No. 3,645,201. None of these patents, however, teach the successful use of a halftone color blend visual effect in high speed printing and all use solid color printing techniques." PA0 "forming a series of line-engraved press plates from said line screen transparencies wherein each press plate corresponds to one of the colors in the original art work and wherein the printed image from each of said press plates is separated from the color image of the other plates by the field color and the printing lines of the respective press plates are parallel to each other and run in the direction of printing." PA0 "Process for transferring overlayed multiple ink patterns from the surface of a release blanket to a receiving surface on a container or other formed article being printed. A substantially transparent film is first formed on the release blanket, with each ink pattern being printed sequentially over this release film. Proper printing on the release film without picking the release film or previously applied ink films is obtained when certain adhesive and cohesive relationships are maintained between the ink films and the release film. An adhesive film may be formed on the article to be printed, or it may be formed over the ink films and the release film on the release blanket. The receiving surface on the article to be printed is brought into contact with the films on the release blanket, with a resulting total transfer of the films on the blanket to the surface of the article." PA0 "A process of four-color halftone printing onto objects having a curved surface by means of dry offset printing which comprises transferring four images of yellow, magenta, cyan and black process inks having a difference of at least 1 in tackiness value from each other from four blocks onto two transfer areas provided on a blanket differently in the circumferential direction of a blanket cylinder in two layers two by two so that an image of an ink having the highest tackiness value and an image of an ink having the secondarily high tackiness value are transferred onto the first transfer area in that order and an image of an ink having the thirdly high tackiness value and an image of an ink having the lowest tackiness value are transferred onto the second transfer area in that order, and then transferring the images on the first transfer area and the images on the second transfer area in order onto the object having a curved surface in layers. Improved four-color halftone printing apparatus for objects having a curved surface is provided with a blanket cylinder having an effective printing surface of twice the size of the printing surface of the objects. The reproducibility of color and images is very excellent and the printing closely resembling the original can be". PA0 "The individual one-color half-tone images print sequentially on the offset blanket cylinder, with the individual dots of sequential colors standing alone, overlapping or registering, as dictated by the color separation of the original photograph. At locations of dot overlap or registry, the colors blend. In this way, a four-color half-tone image, which is a reproduction of the originally photographed object, is obtained in a wet-form, suitable for printing onto a recipient surface, preferably a preformed article, such as, a plastic container."
The O'Connell solution to the above quoted problems is succinctly stated in clause d of claim 1 as:
U.S. Pat. No. 4,035,214 granted July 12, 1977 to Laurence Verlan Shuppert et al and titled: Total Image Transfer Process teaches a radically different process from that of O'Connell as stated in the Abstract:
The Shupper patent also discusses the difficulties of applying conventional multi-color offset printing to formed containers and states that the necessity of precise registration of color patterns greatly slows the handling of the articles.
U.S. Pat. No. 4,241,657, granted Dec. 30, 1980 to Hikoichiro Fujimori and titled Four-Color Halftone Printing Process For Objects Having Curved Surface, provides yet another solution as stated in the Abstract:
U.S. Pat. No. 4,774,885, granted Oct. 4, 1988 to Reinhold Chmielnik and titled Printing Process Overlapping Multi-Color Dot Images provides a process whereby a multi-color halftone image of an original object is formed on preformed plastic containers or other recipient surface by printing such image while wet from an offset blanket cylinder on which the image is formed by overlying a plurality of one-color halftone images. In column 3, lines 7-16, it is stated that: